There's no problem with mounting ext2 filesystems (whether or not they
use ext2 compression) through the loop driver on a
non-ext2-compressed file.

However, you can't mount a loop-back device on an ext2-compressed file,
like:

# chattr +c myfile
# losetup /dev/loop0 myfile

If you want to do this (i.e. create a large block device from a smaller file or
block device) then you should look at DouBle, which was built for this
very purpose. (See section Reference for DouBle.)

For those interested: The reason the loop driver won't work on a
compressed file is that the loop driver uses bmap, i.e. it asks
the filesystem "Which blocks hold this file?" and tries to access that
block directly. Since the file data is stored in compressed format,
there is no correct answer to that question. You'll get a couple of
syslog messages, like so: